1. Field of the Invention
The invention relates to techniques for the reception of insulating mineral fibers, particularly glass fibers, to separate under the fibering machines, fibers and ambient gases, particularly induced gases or gases having been used for drawing these fibers, to produce a mat of mineral wool.
2. Discussion of the Background
An important stage in the production of products with a mineral fiber base such as glass fibers is that of their collection under the fibering machines. This operation has as its object the separation of fibers and the large amounts of gas generated by the fibering by the burners and especially by air induction. This separation is performed in a well-known way by suction through a reception device which is permeable to gases and impermeable to fibers.
An ordinary type of reception device known as belt reception is described, for example, in U.S. Pat. No. 3,220,812, where it is proposed to receive the fibers coming from a series of fibering machines on a single conveyor of the gas-permeable endless belt type, under which a box is placed under partial vacuum or, multiple boxes under independent partial vacuum. In this reception type, the fibering machines can be brought together to the limits of their respective space requirements, which makes possible relatively short lines; this point being significant if it is known that some production lines can reach the number of 9 fibering machines or more, each fibering machine being of a diameter on the order of 600 mm, for example. Moreover, the only limit lower than the base weight (or surface weight) of the felt produced is that dictated by the mechanical strength problems, which therefore allows the production of the lightest products able to be obtained.
However, obtaining heavy products poses many problems, hereafter in this description, heavy products are products whose base weight is, for example, greater than 2.5 kg/m.sup.2 involving glass wool products whose micronaire value is 3 to 5 g, with the exception of dense products obtained by molding and pressing which do not enter directly into the scope of this invention. This problem is easily explained by the fact that the heavier the mat is that is attempted to be produced, the larger the amount of fibers which are deposited on the same surface of the endless belt, and therefore the greater the resistance to the passage of the gases. To compensate this lower permeability, a greater partial vacuum should be exerted, which has the effect of a crushing of the felt by the pressure of the gases, crushing especially notable in the lower part of the felt, which corresponds to the fibers collected in the first place. The mechanical performances of the product especially at the level of recoveries of thickness after compression thereby are poorer. The degradation of the quality of the resulting product is very appreciable as soon as the partial vacuum has to be brought beyond 8000 to 9000 Pa, while in some installations, a partial vacuum of more than 10,000 Pa already is necessary for mats whose base weight is 2500 g/m.sup.2.
To eliminate this drawback, the gases, certainly, can be sucked in only partially to limit the partial vacuum to a value not damaging the felt, but there then occurs a backflow phenomenon of the fibers in the direction of the fibering machines. Apart from the fact that it impairs a good drawing, this backflow of gases causes an increase of the temperature in the fibering hood and therefore a danger of pregelling of the binder, i.e., a polymerization of the binder while the fibers are still in the unit state, which removes from it almost all activity. Moreover, this backflow can cause the forming of rovings, i.e., dense units of agglomerated fibers, which impair the homogeneity of the product, its appearance, and lower its heat resistance.
It also can be attempted to reduce the rate of passage of the gases through the felts by separating the fibering machines from one another. However, the actual gain is very low because the increase of the dimensions of the hood causes an increase of the air induction and therefore the amount of air to be sucked in.
In patent application EP-A-102 385, it has been proposed to separate the reception into two parts each receiving the fibers produced by one fibering machine out of two. The reception then comprises two conveyors oriented toward one another, to assemble the two formed half-felts. This reception type offers the advantage of providing products with a beautiful outside appearance due to the presence on the two faces of sized over skins which improve the mechanical strength of the product. However, the space requirement of the reception is larger than in a traditional reception and, for high base weights, a beginning of polymerization of the binder can occur before the joining of the half-felts which triggers a delamination of the product.
Further, this notion of a subdivision of receptions was developed in U.S. Pat. No. 4,120,676, which proposes to join a reception unit to each fibering machine, the production line thus being designed as a juxtaposition of basic modules each producing a relatively thin felt, the various thin felts being stacked later to form no more than one felt of great thickness.
This modular design makes it possible to keep the fibering conditions constant whatever the product produced may be. However, it assumes that the lightest products are obtained with a line used very widely below its theoretical capability, which is hardly advantageous from an economical viewpoint.
Another example of modularization of the production lines of mineral wool is given by the so-called receptions with drums joined to a lapper. In this case exemplified by U.S. Pat. No. 2,785,728, the reception is performed on elements in rotation of the drum type. A blank of a low base weight is prepared by a reception device facing one or two fibering machines, consisting of a pair of drums turning in opposite rotation whose perforated surface makes possible the suction of gases by suitable devices placed in the drums. The blank is formed between the drums and falls on a vertical plane before being taken up by the lapper, i.e., an oscillating device which deposits it in interlaced layers on a conveyor where the felt of the desired high base weight is obtained.
These modular designs of receptions make it possible in theory to aim at a much wider range of products if a systematic beginning is made by the production of a felt of low base weight.
However, this assumes a larger initial investment with, moreover, the multiplication of auxiliary equipment (suction and washing devices, in particular). Further the means of partitioning the receptions leads to a large spacing of the fibering machines, and exceptionally long production lines are reached as soon as the number of fibering machines is multiplied.
Moreover, the risks of delamination and irregularity of the product prohibit the production of felts of lower base weights. Thus, a lapper calls for a blank of at least 100 g/m.sup.2 below which its mechanical strength would be insufficient particularly to support the movements of the pendulum, and a sufficient number of superposed layers--to have an optimization of the distribution with the same number of layers at any point of the felt.
Further, to operate systematically with the same flow of fiberized mass certainly comes down to being placed under conditions favoring the reproducibility of the parameters of the fiberizing and, by the same reproducibility, their optimization, but it is especially to manage without the extraordinary capability of the fibering machines to operate according to the flows of fiberized material going, for example, from 1 to 10.
Finally, with equal fiber qualities, a product is marketed at a lower price when its base weight decreases. Therefore it does not seem very wise to be placed precisely in the position where the line produces the lowest tonnages.